In the manufacture of semiconductor devices, ion implantation is used to dope semiconductors with impurities. High energy (HE) ion implanters are used for deep implants into a substrate in creating, for example, retrograde wells. Implant energies of 1.5 MeV (million electron volts), are typical for such deep implants. Although lower energy may be used, such implanters typically perform implants at energies between at least 300 keV and 700 keV. Some HE ion implanters are capable of providing ion beams at energy levels up to 5 MeV. A LINAC (LINear ACcelerator) is often used to accelerate the ions to achieve these high energy levels required at the wafer.
A LINAC is a chain of accelerating assemblies (e.g., stages or slices), applied usually in a straight line. When a beam of ions is accelerated by a LINAC, and applied to a semiconductor substrate to implant the ions into the surface of the semiconductor substrate or wafer, we call the process “ion implantation”.
Digital frequency synthesis (DFS) and digital phase synthesis (DPS) are techniques for creating continuous waveforms with high precision and high reproducibility. Their use in communication systems dates from the mid-1970s, and today they are an integral component in nearly every modem at any communication speed. The two methods taken as a whole are frequently called DDS (Direct Digital Synthesis). This powerful method of phase synthesis has also been applied to research type linear accelerators, where it replaced less accurate analog control systems or was incorporated into digital-signal processing (DSP) systems that have accreted significant circuit functionality to simplify and reduce the physical size of the implementation of the LINAC control system.
The control of the many electrode phases in a LINAC used as an ion-implantation process tool in a production environment, however, introduces phase-control challenges not common to research accelerators. For example, the specific set of data representing the operating electrode voltage amplitudes and phases for the entire accelerator system (a “dataset”) may need to be reproduced on multiple tools in multiple locations, and the dataset may need to be applied and brought to a fully operational state on the tool quickly. It is particularly important that this dataset be quickly reproduced on a production LINAC when multiple ion implantation processes are applied to the same substrates (e.g., wafers) in what is commonly called a “chained” implant process.
In addition, because manual calibration methods are presently used, the ability to transport and implement a dataset among two or more otherwise similar LINAC-based ion-implantation machines is affected by the accuracy with which the non-variant “static” component of the phase errors of the many electrode voltages have been removed from the system during calibration. Such manual phase and amplitude calibrations induce a “human factor” of measurement variations during the calibration process generating machine-specific phase delay errors.
Referring to FIG. 1, a typical high energy ion implanter 10 is illustrated, having a terminal 12, a beamline assembly 14, and an end station 16. The terminal 12 includes an ion source 20 powered by a high voltage power supply 22. The ion source 20 produces an ion beam 24 which is provided to the beamline assembly 14. The ion beam 24 is then directed toward a target wafer 30 in the end station 16. The ion beam 24 is conditioned by the beamline assembly 14 which comprises a mass analysis magnet 26 and a radio frequency (RF) LINAC 28. The mass analysis magnet 26 passes only ions of an appropriate charge-to-mass ratio to the LINAC 28. The LINAC 28 includes a series of resonator modules or acceleration stages 28a-28n, each of which further accelerates or decelerates ions from the energy they achieve from prior stages. The accelerator stages are individually energized by a high RF voltage which is typically generated by a resonance method to keep the required average power reasonable.
The linear accelerator stages 28a-28n in the high energy ion implanter 10 individually include an RF amplifier, a resonator, and an accelerating electrode. The resonators, for example, operate at a frequency of, for example, 13.56 Mhz, with a voltage of about 0 to 150 kV peak-to-peak, in order to accelerate ions of the beam 24 to energies over one million electron volts per charge state. As the ion beam 24 travels through the various accelerator modules or stages 28, some of the ions therein are properly accelerated, whereas others are not. Inefficiencies in ion transport are increased by the errors produced by inaccuracies in the phase calibrations of the electrodes as well as the phase synchronization between the electrodes.
It is necessary to precisely control the frequency and phase of each electrode during implantation of high-energy ions onto a workpiece, such as a semiconductor product. It is important in a production environment, that the dataset representing the electrode voltage amplitudes and phases for an accelerator of an ion implantation system be quickly reproduced and be made fully operational on multiple tools in multiple locations. This is particularly important when the dataset is reproduced on a production LINAC during a “chained” implant process. Accordingly, there is a need in the production environment for an improved HE LINAC-based ion implantation device, utilizing the advantages of direct digital synthesis DDS and a means of automatic phase and amplitude calibrations that avoids the need for error prone manual calibration methods.